This invention relates to a continuous casting machine of the type comprising a wheel with a peripheral casting ring and a band covering the ring through a certain arc, the casting ring being supported by a structure which supports the wheel.
In a machine of this type for continuously casting a metal bar of indefinite length, the metal casting ring comprises along its outer perimetral surface a channel covered by the metal band to form a reception throat for the metal, and is supported by a support and centering structure which transmits the drive torque to the ring. The machine is also provided with a suitable cooling system for the inner surface of the casting ring and outer surface of the band. The molten metal, which is poured into the channel and retained by the metal band which adheres to the ring, is cooled while the wheel rotates through a distance equal to the arc covered by the band, and is extracted solidified from the channel before the wheel has made a complete turn.
There are numerous problems encountered in improving the utilisation of a continuous casting machine of the type described.
Among these are problems of thermal origin, i.e. fatigue phenomena of the material forming the ring due to internal stresses induced by the inevitable frequent temperature changes.
In this respect it is clear that any hypothetical section of the casting ring during its rotational movement undergoes a continuous cyclic change in the thermal stresses, due to the fact that the temperature difference between the inner and outer surfaces of said section is a maximum at the moment of reception of the molten metal, and then progressively decreases during cooling and solidification of the metal until it reaches a minimum after completing one turn, before again receiving molten metal.
These thermal stresses are inevitable because they are brought about by the cooling of the metal and the rotation of the ring, and to them are added further stresses of thermal origin induced into the material by temperature differences existing on the inner cooled surface of the ring, these differences being caused by non-uniform cooling of the ring, i.e. by bad distribution of the cooling fluid.
These two main types of stress of thermal origin give rise to the rapid aging of the ring material, decreasing its useful life and therefore making frequent replacement of the ring necessary with consequent considerable slowing down of production and wastage of material and labour.
In seeking more uniform cooling of the ring and therefore a more uniform distribution of cooling fluid, the cooling device which has been most accepted and which has given the best service is that comprising cooling by means of sprays. This device consists of a non-rotating annular header internal and concentric to the casting ring, suitably supplied with cold fluid and comprising on the surface facing the ring a series of spray nozzles arranged to feed jets of fluid on to the surface to be cooled. The header is arranged stationary since a differentiated cooling action is desired for the various sections of the groove arc actually contacting the molten metal.
Generally however, even a device constructed in this manner is not able to provide a rigorously uniform distribution of cooling fluid on the ring surface, as would be required to prevent the formation of poorly or variously cooled zones which would give rise to internal stresses due to surface temperature differences. This occurs either because the jets of cooling fluid are intercepted by parts of the rotating structure which supports the casting ring or because they strike the ring at points relatively distant from each other. The first case occurs in practice in most types of construction and is due to the fact that the system used for anchoring the ring comprises cross members disposed parallel to the axis of rotation of the ring for connecting together the two centering flanges which support the ring, so that only one of said flanges needs be connected to the drive shaft. This arrangement, advantageous from the point of view of simplicity of assembly and replacement of the casting ring, gives rise to considerable disadvantages with regard to heat removal. In this respect, said support cross members, which can only be located in the space between the spray nozzles and the hot surface of the ring, interfere with the cooling jets during rotation of the casting wheel, giving rise to constantly undercooled zones and thus to adjacent zones of different temperatures, which because of different degrees of expansion cause internal stresses which because of their cyclic character lead to rapid aging of the material. Constructional reasons determine the fact that the jets strike the ring at points relatively distant from each other because there is a limit to the distance at which the spray nozzles, disposed on a header of diameter less than the ring, can be spaced apart. It is evident that in this case there are areas of the ring which are cooled to a greater extent because they are struck directly by the jet of cold fluid, alternating with lesser cooled areas which are not struck directly but are lapped by the heated fluid leaving the ring surface. Again, because of the presence of the cross members, the spray nozzles have to be disposed at a large distance from the ring and this leads to a still greater distance between the jets striking the ring, consequently worsening the uniformity of cooling.
The non-uniform cooling action described above, besides slowing considerably the production rate because of the frequent ring replacement requirements, affects adversely the quality of the resultant product, which may exhibit lack of structural uniformity and accordingly inferior mechanical and electrical properties.
The connection of the flanges by the use of cross members is further disadvantageous in that the drive torque for the ring is transferred through a single flange, which becomes subjected to higher stress conditions. When it is considered that the flange is also subjected to generally cyclic stresses of thermal origin at the very areas of attachment of the interconnecting cross members, it will become apparent how the constructional solution described above is in many ways unsatisfactory.
On the other hand, if the cross members were simply removed, the other flange, mounted idly to the stationary part of the machine, would be forced to rotate by the casting ring itself. An arrangement of this type involves the disadvantage of stressing the ring mechanically, which is also subjected to the highest thermal stresses developed in these machines. Accordingly, the operating conditions are made heavier still for the ring and further aggravated by the marked asymmetry of the above cited mechanical stresses, which may lead to unacceptably high distortion of the ring.
With regard to the internal stresses induced by the temperature gradient between the inner and outer surfaces of the casting ring, these stresses cannot be eliminated because they are caused by the necessary cooling of the metal and the rotation of the ring together with the casting wheel. Because of the impossibility of eliminating these internal stresses, it has been considered impossible up to the present time to find an efficient solution to the problem of ring life with respect to these stresses, and the method so far used has rather been one of using for the ring more appropriate materials or more suitable forms for supporting these stresses, compatible with the casting requirements, but this has not led to appreciable results with regard to the life of the ring, which always has to be frequently replaced.